A considerable need exists for mobile and stationary batteries for a wide range of applications, for example for motor vehicles, stationary plants, such as wind turbine installations, and mobile electronic devices, such as laptops and communication devices. Very high demands are placed on these batteries in respect of their reliability, service life and capacity.
A battery cell consists internally of electrochemical elements, which react with sensitivity to external influences such as pressure differences, air or moisture and must accordingly be protected from these as far as possible.
The presence of a so-called lithium-ion conducting salt is thus necessary for intercalation and deintercalation, for example, in lithium-ion cells. Lithium hexafluorophosphate (LiPF6) is used as a lithium conducting salt both for battery cells with a lower charge, for example of the kind which find an application in portable electronic devices, and in battery cells intended for the automotive sector. This conducting salt is extremely reactive to moisture, with the result that hydrogen fluoride (HF) may be produced by hydrolysis.
In particular in automotive applications for lithium-ion storage technology, however, pressure equalization must take place between the interior of the housing for the battery and the surroundings, since otherwise the housing may be compressed by high differences in the ambient pressure or may expand strongly in another case. High differences in pressure are encountered in air transport, when travelling in mountainous regions, as a result of daily fluctuations in pressure and high fluctuations in temperature. In extreme cases, this can result in the destruction of the housing for the battery as a result of extreme mechanical loading.
Therefore, a permanently occurring volume replacement between the interior of the housing and the environment is assured by means of a pressure equalization element. The pressure equalization element may be constituted by a membrane. Since a certain proportion of water vapor is always present in the ambient air, this passes through the membrane and into the housing as a result of volume replacement and diffusion.
A desiccant is generally used in order to prevent the aforementioned problems due to the presence of condensation water in the interior of the housing. However, a desiccant mass of several kilograms over a service life of a number of years is required, even in the case of the smallest housing volumes, or else provision must be made for the replacement of the desiccant.
Described in DE 10 2012 202 103 A1 is a pressure equalization element for a lithium-ion accumulator, in which a membrane is connected in series with two valves arranged parallel and in opposition to each other. In addition, a desiccant may be provided in the pressure equalization element.